The present invention relates to a method of making improved cemented carbide tools for shaping or otherwise working materials. The invention has particular application in making metal working tools, and specifically tools used in the manufacture of tubular casings and similar articles, such as two-piece beverage cans.
A two-piece can is made by a drawing and wall ironing process. In general, a two-piece can is made by stamping out metal discs from a metal plate. A metal “cup” is formed from the disk. The formed cups are pushed through a body-forming die comprising a plurality of annular rings, generally known as draw, redraw, and ironing rings, by a body-forming punch. The clearances between the body-forming punch and the plurality of rings become progressively smaller so that the thickness of cup wall is reduced and the cup is elongated. This process is generally referred to as the ironing operation. It is a particularly demanding operation causing high wear on the tools and is sensitive to the dimensional changes and lubrication conditions. Because of the tremendous volume of beverage cans manufactured each year, each slight improvement in the manufacturing process can result in tremendous savings.
Tools for imparting a desired shape, form, or finish to a material, such as dies, punches, and the like, are characterized by extreme hardness, compressive strength and rigidity. This is particularly necessary when shaping metals or similar materials. Commercial material working tools for mass production must also be resistant to wear, erosion and chipping from repeated and continuous stress and abrasion. In addition these tools should also exhibit good corrosion resistant properties in order not to be damaged by the surrounding liquid media (coolant/lubricant). These tools must also be made from materials which can be designed and machined to close tolerances and maintain dimensional stability over a wide range of operating conditions.
It is known to make punches, dies, deep draw tooling and similar material working tools from a variety of materials, including metals, cemented carbide and conventional ceramics. These known materials all have certain undesirable limitations. When making tools for shaping metal articles, particularly tubular casings such as two-piece beverage cans, the problems of prior known materials becomes particularly significant.
A possible way to achieve better performance in can manufacturing is the use of ceramic materials, e.g. whisker reinforced alumina or silicon nitride as are disclosed in U.S. Pat. No. 5,095,730 and U.S. Pat. No. 5,396,788 respectively, but so far conventional cemented carbide seems to keep its position as the preferred material.
A second possible way to achieve better performance is the use of ultrafine grained cemented carbide. Many improvements could be reached by a grain size reduction leading to better wear resistance as described in EP-A-1726672.
Slightly better corrosion resistance is expected from ultrafine material as also described in EP-A-1726673. However, this improvement is believed to be a result of thinner binder films obtained via the WC grain size reduction. Thus, even if a slight improvement is reached, the leaching mechanism will not be drastically modified, leading to binder elimination with a consequent destruction of the cemented carbide structure.